[Flutter] Create new List when Button is pressed

How am I able to create a new List when im pressing a Button? Is it possible with a loop or sth?

List<List<Countries>> groups = [];
...
child: GestureDetector(
onTap: () {
setState(() {
int numberOfGroups = groups.length;
int nextGroup = numberOfGroups++;
String number = nextGroup.toString();
List<Countries> number = [];
groups.add(number);
});
},


This doesnt work

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